Vertically Expanded Crystalline Porous Covalent Organic Frameworks.

Area of Science:

• Materials Science
• Supramolecular Chemistry
• Nanotechnology

Background:

• Covalent organic frameworks (COFs) are limited by small interlayer distances (3-6 Å) due to π-π stacking, restricting guest access.
• This confinement hinders interactions with the extended faces of COF layers, limiting their application in molecular separation and confinement.
• Current COFs primarily allow guest entry/exit along the z-direction, leaving the x-y plane inaccessible.

Purpose of the Study:

• To develop a strategy for increasing interlayer distances in COFs.
• To create accessible nanospaces within COFs for enhanced molecular interactions.
• To improve the separation efficiency of COFs for specific guest molecules like benzene and cyclohexane.

Main Methods:

• Synthesized 2D covalent cobalt(II) porphyrin layers via topology-guided polymerization.
• Assembled vertically expanded COFs using bidentate axial pillars linked by coordination bonds.
• Utilized vapor sorption, breakthrough experiments, and computational studies to analyze framework properties and separation performance.

Main Results:

• Successfully created vertically expanded COFs by replacing π-π stacks with coordination bonds.
• Achieved discrete interlayer apertures defined by pillar length, opening previously inaccessible interlayers.
• Demonstrated efficient separation of benzene and cyclohexane mixtures under ambient conditions due to enhanced interlayer interactions.

Conclusions:

• Vertically expanded COFs offer a new platform for molecular confinement and separation by creating accessible interlayers.
• The strategy of using coordination bonds to expand COFs enables supramolecular interactions with exposed π planes.
• Optimized interlayer slits in these expanded frameworks enhance selectivity for separating mixtures like benzene and cyclohexane.